The long-term objective of the project is to establish how the T cell receptor (TCR)/CD3 complex transduces signal to the interior of the cell and in particular how this signal is modulated to induced cellular responses appropriate to a T cell's stage in development and mature phenotype. The specific aim is to examine the role of the CD3 components in this process. For this purpose a mouse model has been generated by gene targeting which specifically lacks expression of the CD2 delta component. These animals show a unique phenotype affecting both developing and mature T cells. The effect of the mutation is being examined in detail with respect to 1) mature T cell function, 2) T cell development, and 3) assembly and transport of TCR/CD3 complex. 1) Function of mature alphabeta T cells will be assessed by challenge with live virus, allogeneic cells and Th-dependent antigens. Function of gammadelta T cells will be examined by crossing the CD3 delta mutants to a T22b -specific gamma delta TCR transgenic line and assessing responsiveness to appropriate stimulator cells. 2) T cell development in the mutant is blocked at the double positive stage suggesting a defect in positive selection. Crossbreeding with alphabeta TCR transgenic lines will be carried out to allow examination of thymic selection processes for TCRs of defined antigen and MHC specificity. Initial results from such experiments confirm the suggestion that positive selection is blocked, while indicating that negative selection remains operative. gamma delta TCR transgenics will be similarly used to study positive selection of gammadelta T cells. In order to define elements on the CD3 delta molecule that affect positive selection, we will introduce mutated versions of CD3 into the CD3-delta deficient animals as transgenes. 3) Studies of assembly and transport of TCR/CD3 complex in CD3-deficient animals will focus on determining the reason or the expression of different levels of surface TCR on gamma delta and alphabeta T cells. One hypothesis that will be explored is that surface levels are controlled by cell-specific intracellular control of assembly and/or transport. TCR-mediated signal transduction is central to T cell function. Impairment of T cell function involving disregulation of CD3 component expression has been observed in T cells of tumor-bearing mice and in HIV infected cells. Lack of expression of particular CD3 components can be a specific cause of human immunodeficiency.